Method For Guiding A Downhole Tool Assembly Using An Above-Ground Receiver System

ABSTRACT

A method and receiver system for identifying a location of a magnetic field source using two horizontally displaced tri-axial antennas. In a preferred embodiment two tri-axial antennas are positioned at opposite ends of a receiver frame. Each antenna detects in three dimensions a magnetic field from a source or transmitter. The receiver is maintained in a horizontal plane and the receiver is moved in the horizontal plane until a flux angle measured at each of the two points is zero so that the receiver is in the vertical plane perpendicular to the axis of the source. The depth and location of the source in three dimensions relative to the receiver is determined using the detected field values. The receiver is moved in a direction defined by a line containing the two points of the receiver until a magnitude of the magnetic field detected at each of the two points is substantially the same so that the receiver is positioned above the source.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 13/117,264, filed May 27, 2011, now U.S. Pat. No. 8,482,286, which is acontinuation of U.S. application Ser. No. 11/863,903. filed Sep. 28,2007, now U.S. Pat. No. 7,952,357, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of locatingunderground objects, and in particular to locating and tracking a beaconor transmitter within the field of operation of a horizontal drillingmachine.

SUMMARY OF THE INVENTION

The present is directed to a method for guiding a downhole toolassembly. The method comprises setting a receiver assembly at a targetpoint. The receiver assembly comprises two antenna assemblies disposedin a substantially horizontal plane, such that the two antennaassemblies each lie on a desired borepath. A magnetic field transmittedfrom the downhole tool assembly is simultaneously detected in threedimensions at each of the two antenna assemblies to determine a positionof the downhole tool assembly.

The present invention is further directed to a method for tracking adownhole tool assembly having a transmitter to transmit a magneticfield. The method comprises maintaining two and only two tri-axialantennas of a receiver assembly in a substantially horizontal plane andsimultaneously detecting in three dimensions the magnetic field at eachof the two and only two tri-axial antennas. The receiver assembly ismoved in a horizontal plane until a flux angle measured at each of thetwo antennas is zero.

In yet another aspect the present invention is directed to a receiverassembly for tracking movement of a downhole tool assembly through theground. The downhole tool assembly comprises a transmitter. The receiverassembly comprises a frame, two and only two tri-axial antennassupported by the frame, wherein the two antennas detect a magnetic fieldfrom the transmitter; and a processor. The processor receives an antennasignal from each of the two antennas and determines a location of thedownhole tool assembly relative to the frame using the antenna signals.The two antennas detect the magnetic field when the antennas are held ina substantially horizontal plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a horizontal directional drilling systemfor drilling a horizontal borehole and a tracking system built inaccordance with the present invention.

FIG. 2 is a perspective view of a receiver assembly constructed inaccordance with the present invention.

FIG. 3 is a perspective, partially cut-away vim of an antenna assemblyfor use with the present invention.

FIG. 4 is a block diagram of a portable area monitoring systemconstructed to detect and process signals emanating from a boring tool.

FIG. 5 is a flowchart illustrating a process for improved accuracy indetermination of the location of a transmitter.

FIG. 6 is a geometrical representation of the relationship between atransmitter and a tilted receiver.

FIG. 7 is another geometrical representation of the relationship betweena transmitter and a tilted receiver.

FIG. 8 is graphical representation of the relationship between areceiver and a transmitter for an alternative method of using thereceiver.

FIG. 9 is graphical representation of a preferred embodiment for thedisplay of the receiver when the receiver is used in the method of FIG.8.

BACKGROUND OF THE INVENTION

The horizontal directional drilling (HDD) industry traditionally useswalk-over tracking techniques to follow the progress of a bore orutility installation, to find the surface location immediately above adrill bit or backreamer, and to determine the depth of the drill bit orbackreamer from that surface location. The primary tracking tools are asubsurface transmitter and a hand-carried surface receiver. Thetransmitter, located in or very near a boring tool or backreamer,generally emits a magnetic dipole field created by a single coil dipoleantenna. The transmitted dipole field can be used for both location andcommunication with the above ground receiver.

Conventional receivers often contain an arrangement of three antennasmounted in each of the three Cartesian axes. When the antennaarrangement senses the dipole field, the output of each antenna isproportional to the magnitude of the magnetic flux density as detectedalong the axis of the particular antenna. The signals from the antennasare mathematically resolved to provide information about the relativelocation of the boring tool. The process of locating the dipole, andthus the boring tool, generally involves two steps: determining itslocation along the z-axis (fore and aft) and then along the y-axis (leftand right). One skilled in the art will appreciate a receiver can locatea transmitter in the fore-aft direction (along the z-axis) using theamplitude and phase of the transmitter's generated horizontal andvertical field components as measured in the vertical plane normal tothe surface and extending through the transmitter axis (the x-z plane).In situations where the transmitter is not in a horizontal plane, suchthat the pitch of the transmitter is not 0, the determined position ofthe transmitter may or may not be directly below the receiver. Areceiver can also determine the location of a single transmitter in theleft-right directions using the amplitude and phase of the dipole fieldin the horizontal plane (the y-z plane). However, the left-rightdetermination can only be used either in front of or behind thetransmitter because there is no y component to the dipole field when thereceiver is directly above the transmitter (such that z=0). There iscurrently no satisfactory method of simultaneously locating thetransmitter in both the fore-aft and left-right directions with anantenna arrangement positioned directly over the transmitter.

DESCRIPTION OF THE INVENTION

With reference now to the drawings in general, and FIG. 1 in particular,there is shown therein a horizontal directional drilling system (“HDD”)system 10 for use with the present invention. FIG. 1 illustrates theusefulness of horizontal directional drilling by demonstrating that aborehole 12 can be made without disturbing an above-ground structure,namely a roadway or walkway as denoted by reference numeral 14. To cutor drill the borehole 12, a drill string 16 carrying a drill bit 18 isrotationally driven by a rotary drive system 20. When the HDD system 10is used for drilling a borehole 12, monitoring the position of the drillbit 18 is critical to accurate placement of the borehole andsubsequently installed utilities. The present invention is directed to asystem 22 and method for tracking and monitoring a downhole toolassembly 24 during a horizontal directional drilling operation. Althoughthe invention is shown with the system boring a borehole, the inventionis equally applicable to a backrcaming operation where the borehole isenlarged and prepared for the installation of the desired utility.

The HDD system 10 of the present invention is suitable fornear-horizontal subsurface placement of utility services, for exampleunder the roadway 14, building, river, or other obstacle. The trackingsystem 22 for use with the HDD system 10 is particularly suited forproviding an accurate three-dimensional locate of the downhole toolassembly 24. The locating and monitoring operation with the presenttracking system 22 is advantageous in that it may be accomplished in asingle movement, or a minimum of coordinated movements, to a point abovethe drill bit 18. These and other advantages associated with the presentinvention will become apparent from the following description of thepreferred embodiments.

With continued reference to FIG. 1, the HDD system 10 comprises thedrilling machine 28 operatively connected by the drill string 16 to thedownhole tool assembly 24. The downhole tool assembly 24 preferablycomprises the drill bit 18 or other directional boring tool, and anelectronics package 30. The electronics package 30 comprises atransmitter 32, or magnetic field source, for emitting a signal throughthe ground. Preferably the transmitter 32 comprises a dipole antennathat emits a magnetic dipole field. The electronics package 30 may alsocomprise a plurality of sensors 34 for detecting operationalcharacteristics of the downhole tool assembly 24 and the drill bit 18.The plurality of sensors 34 may generally comprise sensors such as aroll sensor to sense the roll position of the drill bit 18, a pitchsensor to sense the pitch of the drill bit, a temperature sensor tosense the temperature in the electronics package 30, and a voltagesensor to indicate battery status. The information detected by theplurality of sensors 34 is preferably communicated from the downholetool assembly 24 on the signal transmitted by the transmitter 32 usingmodulation or other known techniques.

With reference now to FIG. 2, shown therein is a preferred embodiment ofthe tracking system 22 of the present invention. The tracking system 22comprises a receiver assembly 36. The receiver assembly 36 comprises aframe 38, a computer processor 40, and first and second antennaarrangements 41 and 42 supported by the frame. The processor 40 issupported on the frame 38 and operatively connected to the antennaarrangements 41 and 42. The frame 38 is preferably of lightweightconstruction and capable of being carried by an operator using a handle44. In a preferred embodiment, the receiver assembly 36 also comprises avisual display 46 and a battery 48 for providing power to the variousparts of the receiver assembly. The visual display 46 may be adapted toprovide a visual representation of the tracking system 22 relative tothe drill bit 18, or transmitter 32, and other information useful to theoperator. The receiver assembly 36 may also comprise a transmittingantenna (not shown) for transmitting information from the receiverassembly to the drilling machine 10 or other remote system (not shown).

The antenna arrangements 41 and 42 are supported on the frame 38 andseparated from each other by a known distance and in known relativepositions. Preferably, the antenna arrangements 41 and 42 are positionedon the frame 38 so that when the frame is maintained in a substantiallyhorizontal plane, the antennas will lie in the horizontal plane. Morepreferably, the antennas are separated by a distance of thirty (30)inches. Most preferably, the frame 38 will define an axis between theantenna arrangements 41 and 42. One skilled in the art will appreciate agreater distance or spread between the antennas will provide betterresolution and accuracy. Other receiver configurations are alsopossible, as long as the antenna arrangements 41 and 42 are capable ofisolating the magnetic field in each of the Cartesian axes at the pointon the frame 38 where the antenna is positioned.

Each of the antenna arrangements 41 and 42 is preferably a tri-axialantenna. More preferably, the antennas 41 and 42 are adapted to measurethe total magnetic field at their respective position on the frame 38.Preferably, each antenna 41 and 42 will comprise three orthogonalantennas which measure the magnetic field along their specific axis ofsensitivity. Each of the signals detected by the three orthogonalantenna is squared, summed, and then the square root is taken to obtainthe total field. This calculation assumes the sensitivities of eachantenna are the same and that the center of each antenna is coincidentwith the other two such that the antenna arrangement is measuring thetotal field at a single point in space. Measurements and calculationsare also simplified where the antennas 41 and 42 are oriented or alignedin the same way, relative to each other. However, if the antennas 41 and42 are not pointed in the same direction, adjustments can be made to thecalculations to compensate for the alignment difference.

Referring now to FIG. 3, there is shown therein the preferred embodimentfor the antennas 41 or 42 for use with the present invention. Theantenna 42 comprises a support structure 50 defining three channels 52for each of three receiving coils 54 a, 54 b, and 54 c. The supportstructure 50 is preferably formed of lightweight plastic andmanufactured in such a way that the three channels 52 are eachdimensionally identical. More preferably, the support structure 50 has asubstantially cubical shape and each of the three channels 52 defines arectangular aperture area having a center point. Most preferably, thechannels 52 are mutually orthogonal and oriented so that the centerpoints are coincident. Due to the channel configuration, the coil loops54 all have coincident center points, and their sensitivities aresubstantially identical. An antenna arrangement suitable for use withthe present invention is more fully disclosed in commonly assigned U.S.patent application Ser. No. 11/382,644, the contents of which areincorporated herein by reference. One skilled in the art will appreciateother embodiments for the tri-axial antennas 41 and 42, such as ferriterods or printed circuit boards, may be used.

With reference now to FIG. 4, shown therein is a block diagram of thepreferred embodiment of the receiver assembly 36 of the presentinvention. The antenna arrangements 41 and 42, as described earlier,measure a change in the magnetic field from the source transmitter 32. Achange in the magnetic field sensed will result in a voltage beinginduced in response to the transmitter's 32 magnetic field. The voltagesfrom the antennas 41 and 42 are sent to filters 60 and amplifiers 62.Filters 60 eliminate the effects of other signals received by theantennas 41 and 42 from local noise sources. Amplifiers 62 increase thesignal received by the antennas 41 and 42. An A/D converter 64 is usedto convert analog waveform information into digital data.

The digital data from the A/D converter 64 is then sent to a centralprocessor 66 (CPU) to calculate the location of the transmitter 32relative to the receiver assembly 36. The CPU 66 may comprise a digitalsignal processor (DSP) and a microcontroller. The CPU 66 decodes theinformation from the A/D converter 64 and performs calculations todetermine the location of the transmitter in a manner yet to bedescribed. The CPU 66 may also discern information transmitted on themagnetic field, to determine the battery status, pitch, roll, and otherinformation about the downhole tool assembly 24.

The receiver assembly 36 may also comprise one or more sensors 68 usedto sense operational information about the receiver assembly 36. Forexample, one or more accelerometers, or other known inclination andorientation sensors or magnetic compasses, may provide informationconcerning the roll or tilt of the receiver 36. Information from thesensors 68 is provided to the A/D converter 64 and to the CPU 66 wherethe DSP may make calculations to compensate for the receiver 36 notbeing maintained precisely in the horizontal plane.

In the preferred embodiment the receiver assembly 36 further comprises auser interface 70 having a plurality of buttons, joysticks, and otherinput devices. The operator can input information for use by the CPU 66through the user interface 70. Information entered through the userinterface 70 or determined or used by the CPU 66 may be displayed to theoperator on the visual display 72 screen. The receiver assembly 36 alsocomprises a radio antenna 74 for transmitting information from the CPU66 to a remote unit, such as at the drilling machine 10.

The receiver 36 is preferably powered by a battery assembly 76 and powerregulation system 78. The battery assembly 76 may comprise multipleC-cell sized batteries, though other sources are contemplated, such asrechargeable batteries, solar panels or fuel cells. The power regulationsystem 78 may comprise a line a regulator or switch mode regulator toprovide power to the various components of the receiver 36.

The present invention also contemplates a method and filter arrangementfor improving accuracy and processing of communications and signalsreceived by the antennas 41 and 42. A preferred method of communicationof information from the transmitter 32 is a combination of On-Off Keyed(“OOK”) communication and Differential Phase Shift Keying (“DPSK”)communication. In the preferred method, a synchronization sequenceinvolves turning off the signal from the transmitter 32 for a briefinterval. The receiver 36 is preferably programmed to recognize the‘off’ time in the present scheme as the synchronization period andprovides an opportunity for the receiver to measure the noise floor. The‘off’ time is preferably a sufficient time for the receiver 36 tomeasure the noise floor. The bit rate for the present communicationscheme is preferably very near the optimal bit rate required to senddata from the transmitter 32. Subsequently, traditional DPSKcommunication is preferably used to transfer information from thetransmitter 32 to the receiver 36. The present communication scheme isfurther beneficial because the transmitter 32, with the exception of thesynchronization period, is always transmitting a signal to the receiver36 and the receiver can therefore locate the transmitter substantiallycontinuously.

An example of a transmission with the communication scheme of thepresent invention would comprise a packet of 1 second duration. Thepacket preferably begins with a 50 ms period during which thetransmitter 32 does not transmit and is off. The transmitter 32 wouldthen be turned on for a 50 ms period, during which no phase shifts arepermitted so that the phase reference can be generated and recognized bythe receiver 36. Subsequently, DPSK is used, preferably at 60 bps, totransmit the remaining 54 bits effectively. The receiver 36 willpreferably parse the packet as 10 6-bit non-zero characters. Morepreferably, the first 6-bit character is treated as the synchronizationsequence, followed by 9 6-bit characters of data. The protocol for thecharacters can be structured in any convenient manner to transmitinformation such as roll, pitch, temperature, etc.

In an alternative method of communicating and transmitting informationfrom the transmitter 32, the rotation of the downhole tool assembly 24is ceased and the tool assembly and the transmitter are held at aconstant roll position. Preferably, the tool assembly 24 and transmitter32 are rotated to a predetermined roll angle and allowed to remainstationary for a predetermined amount of time. More preferably, thetransmitter 32 will be allowed to remain stationary for at least fiveseconds. If the transmitter 32 remains at the predetermined roll anglefor the period of time, the transmitter 32 may be programmed to stopcommunication and transmit a simple carrier signal with no modulation.When the transmitter 32 is again rotated, communication of informationon a modulated signal is resumed. Alternatively, when the transmitter 32is maintained in a constant roll position, the transmitter may sendunmodulated roll characters. An unmodulated character would indicate tothe receiver 36 that depth measurements can be taken more accuratelywhile the unmodulated character is being transmitted. The receiver 36also may adjust filter characteristics to change the frequency responsefor the received signal during the transmission of an unmodulatedcharacter.

For improved reception and detection of signals from the transmitter 32,the receiver 36 comprises two parallel sets of digital signal processing(“DSP”) filters 79 (shown in FIG. 4), implemented in the processor 66.Although software implemented DSP filers are discussed here, theinvention also contemplates a hardware filter implementation. A firstset of filters 79 a preferably comprises wide bandwidth filters. Asecond set of filters 79 b preferably comprises narrow bandwidthfilters. The narrow bandwidth filters 79 b will preferably have abandwidth of approximately 10 Hz. The wide bandwidth filters 79 a willpreferably have a bandwidth range of 125-200 Hz, and are used to decodeinformation from a modulated signal and to perform calculations forlocation of the transmitter 32. When the transmitter 32 radiates only acarrier signal, the determination of the location of the transmitter isbased solely on the output of the narrow filters 79 b. The widebandwidth filters 79 a would again be used for location when thetransmitter 32 begins communication and the wide bandwidth filtersdetect the transmitted signal. The wide bandwidth filters 79 a can beused to validate communications, ignoring random noise and identifyingsynchronization characters.

Referring now to FIG. 5, there is shown therein a flowchart illustratingthe use of the multiple DSP filters 79 a and 79 b by the processor 66.The signal from the transmitter 32 is depicted as reaching the filtersat 400. The signal is received by the narrow filters 79 b at 402 and thewide filters 79 a at 404. The narrow filters 79 b retrieve location dataonly at 406. The wide filters 79 a retrieve location data at 408 anddecoded information at 410. If decoded information is found to bepresent at 412, the processor 66 in the receiver 36 recognizes that thelocation data from the wide filters 79 a at 408 is to be used at 414. Ifno information is found at 412, the location data from the narrowfilters 79 b at 406 is used by the processor 66 at 414. One skilled inthe art will appreciate the use of two sets of filters as describedherein provides the benefit of having more signal noise removed suchthat the accuracy of location measurements will be improved.

The receiver assembly 36 of the present invention uses the magneticfield measurements from the antenna arrangements 41 and 42 to directmovement of the receiver. By appropriate movement, the receiver 36 mayaccurately locate the transmitter 32 in three-dimensional (3-D) spacerelative to the receiver. Each antenna arrangement 41 and 42 obtainsthree distinguishable orthogonal components of a magnetic fieldavailable at any position. In the /preferred embodiment described above,the three antennas each arrangement 41 and 42 provide those magneticfield measurements.

The receiver assembly 36 may be used to locate the transmitter 32 inthree-dimensional (3-D) space. When in the area of the transmitter 32,the receiver 36 is used to find the transmitter plane (the y-axis, wherez=0 by using the flux angles as measured at the antennas 41 and 42. Whenthe flux angles are 0 in both of the antennas 41 and 42, the receiver 36is positioned in the transmitter plane and the location of thetransmitter 32 relative to the receiver can be determined. One skilledin the art will appreciate the flux angle measurement of 0 indicates theflux angle at the antennas 41 and 42 is the same as the pitch of thetransmitter 32 with respect to the horizontal plane; indicating that thereceiver 36 is in the transmitter plane (y-axis, where z=0). Preferably,the location of the transmitter 32 is determined with the transmitter atthe origin of the x-y-z coordinate system. For purposes of thisinvention, the z-axis is designated as being along the axis of thetransmitter 32, the y-axis is designated as the horizontal axisperpendicular to the transmitter's axis, and the x-axis is designated asthe vertical axis perpendicular to the transmitter's axis. Thus, thez-axis is a measure of fore-aft, the y-axis is a measure of left-right,and the x-axis is a measure of depth.

In the preferred embodiment, and particularly in situations when theoperator has a general idea of the location of the boring tool 24 andthe borepath 12, the receiver 36 is used as follows. The receiverassembly 36 is held with the axis between the antenna arrangements 41and 42 substantially perpendicular to the suspected borepath 12.Preferably, the receiver 36 is also held in a substantially horizontalplane, though calculations may be used to compensate for any tilt of thereceiver. More preferably, the receiver 36 will be allowed to deviatefrom the horizontal plane by no more than twenty degrees (20°). Thereceiver 36 is then advanced along the suspected borepath 12 until theflux angle measurement with respect to each antenna 41 and 42 is zero(0).

With the receiver 36 in the transmitter 32 plane, the location of thetransmitter 32 relative to the receiver can be determined using thegeometry of FIG. 6 and known equations. From the measurements andcalculations, for example, the operator can be provided with theleft-right offset distance of the receiver 36 to a point above thetransmitter 32. The receiver 36 can also be moved left or right untilthe total fields measured by the antennas 41 and 42 are the same. Whenthe total fields measured by the antennas 41 and 42 are the same and theflux angle measurement with respect to each antenna 41 and 42 is zero,the receiver 36 will be located directly above the transmitter 32 (atthe point where y=z=0). Repeating slight movements of the receiver 36 toensure the null field measurements (magnetic field measurements takenalong the x-axis) are zero and the total field measurements aremaximized will allow a more accurate indication of the position of thetransmitter 32. Accurate depth determination of the transmitter 32 canbe made when the receiver 36 is directly above the transmitter. In thepreferred embodiment, the signal status or magnitude of the antennas 41and 42 is communicated to the operator via the display 72. Antenna 41and 42 information may be communicated in numerical format or with othergraphical techniques, such as a virtual bubble level.

As previously discussed, the receiver 36 may contain sensors 68 toaccount for tilt or pitch of the receiver and enable the calculation ofβ. Calculations may also be used to compensate for the pitch of thetransmitter 32. Also, as discussed in the procedure above, the value ofy will be 0 where the total field magnitudes in the antennas 41 and 42were found to be equal. However, one skilled in the art will appreciatethat the value of y may be determined from the geometry if the fieldswere not balanced or if the receiver 36 was moved slightly in subsequentsteps.

The location of the receiver 36 relative to the transmitter 32 can beaccomplished with direct solution of the field equations. The magneticfield equations for the system are:

$B_{T,i} = {k \cdot \frac{\sqrt{{3\; z_{i}^{2}} + r_{i}^{2}}}{r_{i}^{4}}}$and $B_{x,i} = {3\; {k \cdot \frac{x \cdot z_{i}}{r_{i}^{5}}}}$ wherer_(i)² = x² + y_(i)² + z_(i)².

From FIG. 7, illustrating the relationships between y_(i) and z_(i), wehave the following equations:

$y_{1} = {y + {\frac{L}{2} \cdot {\cos (\gamma)}}}$$y_{2} = {y - {\frac{L}{2} \cdot {\cos (\gamma)}}}$$z_{1} = {z + {\frac{L}{2} \cdot {\cos (\gamma)}}}$$z_{2} = {z - {\frac{L}{2} \cdot {\cos (\gamma)}}}$

The system of equations can now be solved to determine the location ofthe transmitter 32 relative to the receiver 36. In solving theequations, indiscretions in the signs of y and z may be discovered.However, the indiscretions in these signs can be found by turning thereceiver 36 to determine which antenna arrangement 41 or 42 is detectingthe greatest total magnetic field magnitude. If the receiver 36 isturned along the z-axis, the field magnitude reading can be used todetermine if the receiver is in front of or behind the transmitter 32.Turning the receiver 36 along the y-axis, the field magnitude readingcan be used to determine if the receiver is to the left or right of thetransmitter 32. This information can be provided to the receiver 36 sothat appropriate calculations can be made. Alternatively, a procedurefor use of the receiver 36 can be proscribed to indicate when thereceiver is being held on the y-axis or z-axis so that the receiver canmake the determination of the appropriate signs for y and z in the abovesystem of equations.

The present invention also contemplates a novel technique forcalibrating the receiver 36 to the transmitter 32 where a calibrationconstant k is required. Preferably, the receiver frame is placed on theground with the axis between the antennas 41 and 42 perpendicular (alongthe z=0 plane) to the transmitter 32 axis. The distance the receiver 36is placed from the transmitter 32 need not be specified. Magnetic fieldmeasurements are then taken by the antennas 41 and 42. Using knownmagnetic field equations and the known distance between the antennas, aconstant k can be determined for use with the receiver in subsequentmeasurements.

With both antennas 41 and 42 perpendicular to the transmitter 32 axisand separated by some distance L, the measured field at each antenna podcan be written as

${B_{f} = \frac{k}{\left( {d + L} \right)^{3}}},$

and where B_(u) is the field measured at the antenna nearest to thetransmitter and B_(j) is the field measured at the antenna furthest fromthe transmitter. The distance, d, from the nearest antenna 41 or 42 tothe transmitter 32, can be solved for using the following equation:

$\frac{L \cdot \sqrt[3]{\frac{Bf}{Bn}}}{\left( {1 - \sqrt[3]{\frac{Bf}{Bn}}} \right)} = d$

The constant k can then be calculated by again using the magnetic fieldequations for B_(n) or B_(f).

In an alternative embodiment, the receiver 36 of the present inventioncan be used to find the location of the transmitter 32 even if thegeneral location of the transmitter or the borepath 12 is not known. Forlocating the transmitter 32 in such a situation, the receiver 36 isfirst rotated in a horizontal plane until the signal strength receivedat each antenna 41 and 42 is the same. The receiver 36 is then rotated90° and moved in the direction defined by the axis between the antennaarrangements 41 and 42 until the signal strength received at eachantenna 41 and 42 is again the same. At this point, the receiver 36 isproximate a point above the transmitter 32 and the procedure describedabove for locating the transmitter when the transmitter location andborepath 12 are generally known can be used to pinpoint the location ofthe transmitter.

The present invention can therefore be used to identify the exactcoordinates of the receiver 36 relative to the transmitter 32 using themagnetic field measurements from the plurality of antenna arrangements41 and 42 and the procedures and equations above. The informationconcerning the location of the transmitter 32 is preferably provided tothe operator using the visual display 72. The processes described hereinallow the receiver assembly 36 to be used to locate the downhole toolassembly 24 and transmitter 32 quickly and accurately, with few stepsand little computation.

With reference now to FIG. 8, an alternative embodiment for use of thepresent invention is shown. In the alternative embodiment, the receiver36 is contemplated for use as a target for the boring tool assembly 24.As shown in FIG. 8, the receiver 36 is placed on the ground at a targetpoint in front of the tool assembly 24. Preferably, the receiver 36 ispositioned so that each antenna assembly 41 and 42 is on the desiredborepath 12. With information provided by the receiver 36 for use by theoperator, the operator can direct the tool assembly 24 in order to boreto or through the point where the receiver is positioned or a pointdetermined by the receiver, such as a point at a particular depth belowthe receiver. Using the receiver 36 as a target can also provebeneficial for guiding the downhole tool assembly 24 to a desired pointafter the tool assembly has been steered on a dead-reckoning courseunder an obstacle, such as a building, river, or road, where precisetracking of the borepath has not been possible,

The information provided to the operator preferably includes left/rightsteering guidance, current depth (xc), projected depth (xp) at thetarget point where the receiver 36 is positioned, and horizontal offsetdistance (z0) to the receiver. For determining the left/right direction,the flux angles of the fields measured by the antennas 41 and 42 may beused. With information provided to the operator, the operator can steerthe tool assembly 24 in the direction of the flux angle and the assemblywill begin moving toward the vertical plane containing the receiver, andconsequently toward the borepath 12.

Using the geometry shown in FIG. 8, the position of the downhole toolassembly 24 in a vertical plane containing the borepath 12 can becalculated and provided to the operator. The position of the downholetool assembly 24 preferably comprises a depth (xc) of the downhole toolassembly and offset horizontal distance (z0) to the downhole toolassembly. Preferably, the receiver 36 may assume that the drilling tool24 is in the vertical plane of the desired borepath 12, thus makingcalculations simpler and within reasonable accuracy. The followingformulas can he used to provide the desired information:

Solve for θ from:

${\phi = {{\tan^{- 1}\left( \frac{B_{x}}{B_{z}} \right)} = {\tan^{- 1}\left( \frac{3\; \sin \; \theta \; \cos \; \theta}{{3\; \cos^{2}\theta} - 1} \right)}}},$

-   -   where φ is calculated as the arctangent of the signal strengths        of the antennas 41 and 42 (rotated due to the pitch of the        transmitter 32 and rotation of the receiver 36).

Current depth: X_(c)=r sin(θ+P)

Projected depth: X_(p)=X_(c)−r·cos(θ+P)·tan(P)

Horizontal offset: Z₀=r·cos(θ+P)

With two antennas 41 and 42 available, the receiver 36 car visuallyprovide for the operator an indication of the downhole tool assembly's24 progress toward, and ultimately away from, the receiver. In thepreferred embodiment, the information is graphically displayed at thedisplay 72. With reference now to FIG. 9, there is shown therein arepresentation of a preferred embodiment for the display 72 of thereceiver 36. The display 72 comprises a solid horizontal line 80 thatforms part of a crosshair on the display to indicate the projected depthof the transmitter 32 when the transmitter, or downhole tool assembly24, reaches the target point. A dashed horizontal line 82 is indicativeof the transmitter's 32 current depth. Vertical clashed lines 84 showthe horizontal offset of the transmitter 32 from the receiver 36. Theleft/right direction, and magnitude if desired, is shown by a scaledarrow 86 along a middle portion of the crosshair. Other statusinformation such as transmitter 32 pitch, roll position, and batterystatus, tool assembly 24 temperature, and receiver 36 battery status mayalso be provided for operator information and use.

Various modifications can be made in the design and operation of thepresent invention without departing from its spirit. Thus, while theprincipal preferred construction and modes of operation of the inventionhave been explained in what is now considered to represent its bestembodiments, it should be understood that within the scope of theappended claims, the invention may be practiced otherwise than asspecifically illustrated and described.

1. A receiver system for identifying a location of a magnetic fieldsource, the receiver comprising: a first triaxial antenna to detect adipole magnetic field from the magnetic field source in threedimensions; a second triaxial antenna to detect the dipole magneticfield from the magnetic field source in three dimensions simultaneouslywith the first triaxial antenna; and a processor to receive an antennasignal from each of the first triaxial antenna and the second triaxialantenna and to determine the location of the magnetic field source usingthe antenna signals; wherein the first triaxial antenna and the secondtriaxial antenna are laterally displaced from each other a distance. 2.The receiver system of claim 1 further comprising an orientation sensorto detect an orientation of the first triaxial antenna and the secondtriaxial antenna.
 3. The receiver system of claim 2 wherein theprocessor uses the orientation of the first and second triaxial antennasto determine the location of the magnetic field source.
 4. The receiversystem of claim 1 wherein the first triaxial antenna comprises: a firstwinding defining an aperture area; a second winding defining an aperturearea; and a third winding defining an aperture area.
 5. The receiversystem of claim 4 wherein the aperture area of each winding is the sameand the windings have a common center point.
 6. The receiver system ofclaim 1 wherein the first triaxial antenna is laterally displaced fromthe second triaxial antenna in substantially the same horizontal planeand a known distance.
 7. The receiver system of claim 1 wherein thefirst triaxial antenna and the second biaxial antenna are maintained ina substantially horizontal plane.
 8. A receiver system for identifying alocation of a magnetic field source, the receiver comprising: twotriaxial antennas to detect a magnetic field from a source; and aprocessor adapted to receive an antenna signal from each of the twotriaxial antennas and to determine a location of the source using theantenna signals; wherein the two triaxial antennas detect the magneticfield when the antennas are held in a substantially horizontal plane. 9.The receiver system of claim 8 further comprising a frame to support atleast one of the two triaxial antennas, wherein the location of thesource is determined relative to the frame.
 10. The receiver system ofclaim 9 further comprising supporting the processor on the frame. 11.The receiver system of claim 8 wherein at least one of the two triaxialantennas comprises: a first winding defining an aperture area; a secondwinding defining an aperture area; and a third winding defining anaperture area.
 12. The receiver system of claim 8 wherein the twotriaxial antennas are maintained in substantially the same horizontalplane.
 13. The receiver system of claim 8 further comprising a radioantenna to transmit information from the receiver to a remote unit. 14.The receiver system of claim 13 wherein information transmitted to theremote unit comprises the location of the source.
 15. The receiversystem of claim 13 wherein the remote unit comprises a drilling machine.16. A method for tracking a below ground source of a magnetic field, themethod comprising: maintaining a first triaxial antenna and a secondtriaxial antenna in a substantially horizontal plane; simultaneouslydetecting in three dimensions a magnetic field from a source both thefirst triaxial antenna and the second triaxial antenna; moving one orboth of the first and second triaxial antennas in the substantiallyhorizontal plane until a flux angle measured at each of the first andsecond triaxial antennas is zero; and calculating a location of thesource in three dimensions using the detected magnetic fields.
 17. Themethod of claim 16 further comprising providing a frame to support atleast the first triaxial antenna and a processor used to calculate thelocation of the source in three dimensions.
 18. The method of claim 17further comprising transmitting information wirelessly from the frame toa remote location.
 19. The method of claim 18 further comprisingdisplaying the location of the source at both the frame and the remotelocation.
 20. The method of claim 16 further comprising determining anorientation of the source and using both the orientation and thedetected magnetic fields to calculate the location of the source.